The field of the invention is the measurement of the effects on tissue during cryosurgical operations, and more particularly, the depiction of frozen tissue during such operations.
The primary use of cryosurgery is to ablate tissue in situ by the application of extreme cold to the tissue. Cell death results and the dead cells typically slough. This process is referred to as "cryonecrosis". The use of cryonecrosis procedures has distinct advantages over other surgical methods including: little or no bleeding; lessened need for anesthesia; faster recovery time; lack of scarring; and preservation or selective alteration of structural components of tissue such as collagen. The cryonecrosis procedures lend themselves well to minimally invasive surgery such as percutaneous, endoscopic, and endovascular surgery. They also can be performed more frequently on an out-patient basis, and can be performed on high-risk patients who could not withstand traditional surgery.
Cell death is achieved in cryosurgery through the removal of heat by application of extremely cold temperature, either directly to the tissue, as in the application of a swab or spray of liquid nitrogen to the skin, or through contact with a very cold instrument, the "cryoprobe". Cryoprobes such as those described in U.S. Pat. Nos. 4,946,460; 348,369; 5,334,181; 5,916,212; 4,483,341, typically are powered by liquid or gaseous coolants or a mixture of both. In a typical application, nitrous oxide is delivered to an expansion chamber within the tip of the cryoprobe, where extremely cold temperatures result from the Joule-Thompson effect. The cold temperatures spread to the tip and then to the surrounding tissue.
Heat is extracted from the surrounding tissue in a "heat sink" effect. The tissues freeze first at the surface of the cryoprobe, and an "ice ball" of frozen tissues grows outward from the cryoprobe surface as the heat is extracted. Although this ice ball may be seen, either with the eyes or through medical imaging systems, the boundaries of the ice ball do not accurately measure the region of cryonecrosis. A long-standing problem in this art is the accurate detection and depiction of cell death so that the surgeon can control the cryonecrosis procedure.
The measurement of tissue temperature (thermometry) is an accepted method of predicting cryonecrosis. However, thermometry has severe shortcomings. The cryonecrotic range of temperature is wide, imprecise, and variable from tissue to tissue. It is well known that cryonecrosis can occur at temperatures considered as non-lethal. On the other hand, some tissues are very cryoresistant. Due to this uncertainty, "overkill" is usually built into cryosurgical procedures. The usual recommendations for a standard destructive cryosurgical application, when addressing a cancerous lesion for example, are quick freezing, slow thawing, and repetition of this freezing/thawing cycle until a tissue temperature at least -40 degrees Celsius is measured at the lesion boundary. When this is done, the visible ice ball extends well beyond the boundary of the lesion. Furthermore, tissular thermometry is an invasive method, requiring the insertion of measuring devices, usually in the form of thermocouple needles. The cryoprobe itself provides no information on the temperature of the surrounding tissue and temperature measurements provide only point-specific information. Methods such as those described in U.S. Pat. Nos. 5,433,717 and 5,706,810 have been proposed for producing temperature maps using magnetic resonance imaging (MRI) systems, but such methods are very expensive due to the high cost of the MRI system.
Other methods have also been proposed for predicting cryonecrosis, but all of these image the boundary of the ice ball and do not detect the region inside the ice ball where cell death actually occurs. Such known methods as heat flux measurements, CAT scanning, sonography, therefore exaggerate the boundary in which cell death occurs.
Another such method is bioelectrical impedancemetry which measures the electrical impedance of the ice ball. Close correlations have been found between the impedance of the ice ball and the cryodestructive tissue temperatures. As described in U.S. Pat. No. 4,252,130, when a certain amount of heat is extracted from a biological system, there is a change of phase or change of state which converts the freezable water into ice and has the result of "extracting" from the cell the water of solvation and the "structural" water, in particular the membranous water. Since the stability of a biological system is dependent on the maintenance of an exact concentration of aqueous solutions, the consequences of the loss of water in the crystalline structure of newly formed ice is substantial.
Impedancemetry or bioelectrical tissue impedance measurement detects the moment water freezes as an increase in electrical impedance of the tissue. At least two electrodes are employed to measure the impedance across the target tissues (e.g. a tumor) and to provide an indication to the surgeon when that impedance suddenly increases. Such prior impedancemetry methods have some of the shortcomings of thermometry in that their ability to accurately predict cryonecrosis was limited. In addition, as exemplified by U.S. Pat. Nos. 4,140,109 and 4,306,568 many prior procedures are invasive in that they require the implantation of separate needle electrodes or other sensors.